.back-top {
  width: 40px;
  height: 40px;
  border-radius: 20px;
  background-color: rgba(0, 0, 0, .7);
  position: fixed;
  bottom: 100px;
  right: 20px;
  z-index: 900;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  color: #ffffff;
  transition: all 1s;
  opacity: 0;

  &.show {
    opacity: 1;
  }
}
